It is known that polypropylene can be used in melt spinning such as spun bonding. Polypropylene for this use is usually subjected to chemical treatment with a considerable amount of a peroxide in an additive stage following polymerization, which is called a controlled rheology treatment (hereinafter abbreviated as CR treatment). Without the CR treatment, polypropylene is unsuited to high-speed spinning because the spun filament would break even at a low take-up speed.
However, the CR treatment incurs an increase of cost of production, and the reaction between the polymer and the peroxide involves by-production of low-molecular weight fragments and color change of the polymer. In particular, the low-molecular fragments not only cause fuming on spinning, which is unfavorable to the production process, but stick to the molding machine to seriously reduce the productivity.
Polypropylene prepared by polymerization in the presence of a metallocene catalyst is known usable in high-speed spinning the CR treatment with a peroxide. However, polymerization of propylene using a metallocene catalyst tends to be accompanied with production of trace amounts of high-molecular components which also cause filament failure in high-speed spinning.